Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer that frequently carries an integrated copy of Merkel cell polyomavirus (MCPyV) and expresses viral transforming antigens (TAgs) that likely play a key role in tumorigenesis. MCC tumor cells also express transcripts detected in normal, post- mitotic Merkel cells residing in skin, including a set of mRNAs encoding lineage-specific transcription factors implicated in neuroendocrine cell fate. Work from our lab and others established that MCPyV small T antigen (sTAg), +/- truncated large T antigen (tLTAg), is sufficient to drive transformation in vivo, but MCC-like tumors were not detected in any of these models despite TAg targeting to various potential tumor progenitor. The lack of a viable mouse model of MCC has been a major impediment to progress in this field. Normal Merkel cells arise from epidermal progenitors in specialized cellular compartments called touch domes, but the cell of origin of MCC is unknown. In an effort to override the apparent resistance of multiple cell types to MCPyV TAg-driven MCC development in genetically-engineered mice, we generated mice co- expressing the Merkel cell transcription factor Atoh1, together with MCPyV TAgs, in epidermal keratinocytes. These mice developed small collections of proliferating MCC-like tumor cells, and when coupled with deletion of one copy of p53, yielded gross tumors strikingly similar to human MCCs based on multiple criteria. Given the pivotal role of Atoh1 in MCC tumorigenesis we performed complementary loss-of-function studies, and discovered that Atoh1 knock-down converts human MCC cells from their typical neuroendocrine phenotype and growth in suspension, to cells with adherent growth, loss of neuroendocrine markers, and more aggressive behavior in vivo. We thus hypothesize that Atoh1 and other transcription factors governing Merkel cell fate play a pivotal role in MCC pathogenesis as well as maintenance of the neuroendocrine tumor phenotype. We propose the following aims to test this hypothesis. 1) Generate and characterize MCC-like tumors driven by MCPyV TAg expression targeted to candidate MCC progenitor cells. 2) Determine whether MCPyV TAg expression is required for mouse MCC maintenance. 3) Examine the role of Merkel cell lineage transcription factors in governing neuroendocrine cell fate and biological behavior of human MCC cell lines. These studies will yield new insights into the molecular basis of MCC and validate a much-needed mouse model of MCC.